1. Field of the Invention
The present invention relates to a dose distribution reading method and reader for a glass dosimeter, in which a two-dimensional or three-dimensional dose and dose distribution of the glass dosimeter are simultaneously read using a two-dimensional camera.
2. Description of the Related Art
In recent years, radiation therapy devices such as a gamma knife and a cyber knife have been used in radiation therapy. These radiation therapy devices utilize, a narrow beam gamma ray, which needs to be correctly applied to a lesion part in a concentrated manner. Therefore, it is important to perform inspections such as CT and MRI before performing the therapy and to correctly confirm a radiation dose or an irradiation position.
Additionally, the lesion part to which the radiation therapy is applied is a part in which a surgical operation is difficult, such as the inside of skull, and therefore it has been difficult to confirm an actual beam profile and an irradiation dose. At present, a film photosensitive to gamma rays has been used in this type of irradiation dose measurement. However, this type of film is not suitable for high-dose measurement in which concentrated irradiation is performed in the radiation therapy, and there have been disadvantages that the distribution is observable but a correct dose measurement is not possible.
Moreover, in recent years, a radio chromic film called Gafchromic has been used. In this film, a photosensitive function is not used as in a silver salt film, but a discoloration function (change to blue) which is proportional to an ionizing radiation dose is used, and there is an advantage that developing in a darkroom is not required. However, this radio chromic film has problems in that sensitivity changes in accordance with a storage temperature, and it is not suitable for accurate dose measurement. Furthermore, it is expensive, cannot be reused, and is therefore economically disadvantageous.
On the other hand, a glass dosimeter has heretofore been known as a meter which has a high measurement precision and which is superior in cost. The glass dosimeter comprises a fluorescence glass element formed of phosphate glass containing silver ions. When the fluorescence glass element is activated by exposure to ionized radiation, and subsequently excited with ultraviolet rays, fluorescence is generated from a predetermined glass surface. Since fluorescence intensity is proportional to the exposure radiation dose, the radiation dose can be obtained from the fluorescence intensity.
The above-described glass dosimeter can be reused because it can be reset by thermal treatment, correct measurement is possible even for high doses, and therefore application to the above-described radiation therapy device is considered. In one of the considered use modes of the glass dosimeter in radiation therapy, the fluorescence glass element is disposed in an irradiation position of the radiation therapy device such as the gamma knife or the cyber knife (position where the lesion part is to be present during the therapy), radiation is performed, and it is confirmed whether or not a predetermined dose is applied to an irradiation position obtained from inspections such as CT and MRI beforehand.
As a concrete example of a conventional glass dosimeter, there is a dosimeter disclosed in Jpn. Pat. Appln. KOKAI Publication No. 3-102283. This has been developed for the purpose of obtaining a radiation quality and an incident direction of radiation in a personal dosimeter from the standpoint of exposure accident analysis, and a fluorescence detection position (or area) is changed by a diaphragm to detect the fluorescence. The incident direction is estimated by the use of characteristics that a peak position of an exposure dose deviates in accordance with the incident direction by the use of a dosimeter element having a slit in its middle (filter absence portion) as shown in FIG. 26 of the publication (fluorescence intensity distribution known with a dosimeter having this structure is a one-dimensional distribution). As this device is used for whole body exposure, and not for narrow beam irradiation, correct dose distribution measurement, which is an object of the present invention, is impossible.
Moreover, a radiation dose reader capable of detecting a fluorescence intensity distribution is described in Japanese Patent No. 3014225. In this reader, area sensors such as CCDs are used in a fluorescence detector in the dose reader for the purpose of estimating the radiation incident direction from the position of a fluorescence peak in the glass device in which a filter is disposed in the same manner as in the Jpn. Pat. Appln. KOKAI Publication No. 3-102283. Additionally, it is impossible to obtain a sufficient sensitivity with the CCD camera at the time of the filing of the application. Therefore, the reader is constituted in such a manner that the detector is brought into close contact with the glass device (Claim 1 of the publication) and that an electronic multiplier plate is disposed between the detector and the glass device (Claim 2 of the publication), and it has been difficult to obtain a fluorescence intensity distribution fine enough to correspond to a pixel of the CCD.
Further, in Jpn. Pat. No. 3057168, a technique is described in which a fluorescence intensity fluctuation by an output fluctuation of an ultraviolet excitation light source is corrected in a fluorescence glass dosimeter measurement apparatus using a nitrogen gas laser as a light source, but this technique relates to the fluctuation correction of a total amount of exciting ultraviolet radiation received by the fluorescence glass element.
However, the above-described technique has the following problem. That is, since only the total amount of the fluorescence intensity is detected by a photomultiplier tube or the like in the conventional glass dosimeter, two-dimensional and three-dimensional dose distributions cannot be read, and it has been impossible to obtain the irradiation range and radiation dose by the radiation therapy device. Even when the radiation incident direction can be estimated, the filter has to be also used in the fluorescence glass element, and it has not been possible to read the dose distribution for narrow beam irradiation, which is an object of the present invention. Especially, since the fluorescence glass element usually has a thin plate or film form, it is difficult to securely set the device on a plane including a beam concentrated point. Therefore, there has been a demand for development of a technique capable of reading a three-dimensional dose distribution to obtain the dose and dose distribution at the point where the beam is concentrated, and its periphery.